The polyphenylene ether resins comprise a class of thermoplastic materials characterized by outstanding physical properties, including hydrolytic stability, excellent dielectric properties, a broad temperature use range and dimensional stability at elevated temperatures. They can be made by a variety of catalytic and non-catalytic processes from the corresponding phenols or reactive derivatives thereof. In general, they are prepared by the oxidative coupling of a phenolic compound with a complex copper catalyst. By way of illustration, descriptions of the preparation of polyphenylene ether resins are contained in Hay, U.S. Pat. No. 3,306,874 and 3,306,875 and in Stamatoff, U.S. Pat. No. 3,257,357 and 3,257,358, the disclosures of which are incorporated herein by reference.
In the Hay patents, polyphenylene ether resins are prepared by an oxidative coupling method comprising passing an oxygen-containing gas through a reaction solution of a phenol and a metal-amine complex catalyst. In the Stamatoff patents, polyphenylene ethers are prepared by reacting the corresponding phenolate ion with an initiator, e.g., an acid peroxide, in the presence of a complexing agent.
In Cizek, U.S. Pat. No. 3,382,435, incorporated herein by reference, it is disclosed that polyphenylene ethers and styrene resins are combinable in virtually all proportions to provide compositions having many properties improved over those of either of the components.
It has been proposed that compositions of polyphenylene ether resin and polystyrene can be modified by the inclusion of block copolymers of the vinyl aromatic compound, e.g., styrene, and a conjugated diene, e.g., butadiene. Such compositions, which have excellent impact resistance, are disclosed in U.S. Pat. No. 3,994,856 assigned to the same assignee as herein. Compositions of a polyphenylene ether resin, a styrene resin and a block copolymer of an aromatic hydrocarbon such as styrene and a conjugated diene such as butadiene which has had its unsaturation reduced to less than 10% of its original value have also been proposed. These are disclosed in U.S. Pat. No. 4,167,507 and assigned to the same assignee as herein.
In formulating the aforementioned thermoplastic compositions, it has generally been thought that the use of polyphenylene ether resins of relatively high molecular weight, i.e., 16,000 or greater, are essential if good impact strengths are to be obtained in the finished molded article. Such high molecular weight polyphenylene ether resins are characterized by having polymer chains of 130 units or more and intrinsic viscosities, as measured in chloroform at 30.degree. C., of at least about 0.4 deciliters/gram or more.
The physical properties of polymers, in general, are very dependent on molecular weight. It has been observed for example, that the impact strength of a polyphenylene ether resin, as measured by Notched Izod impact, decreases in proportion to a decrease in the intrinsic viscosity and, correspondingly, the molecular weight of the polyphenylene ether. By way of illustration, for a poly(2,6-dimethyl-1,4-phenylene ether)resin having an intrinsic viscosity of about 0.46 deciliters/gram in chloroform at 30.degree. C., a Notched Izod impact of 4.2 ft.lbs./in. is obtained. On the other hand, for a poly(2,6-dimethyl-1,4-phenylene ether)resin of 0.27 deciliters/gram intrinsic viscosity, the Notched Izod impact is only 1.0 ft.lbs./in.
It has now been surprisingly discovered that when a low molecular weight polyphenylene ether resin, i.e., having an intrinsic viscosity of less than 0.4, preferably 0.33, deciliters/gram in chloroform at 30.degree. C., and a styrene resin are combined with an elastomeric block copolymer of a vinyl aromatic compound and a conjugated diene, the resulting compositions possess good impact strengths. The results are unexpected in view of the strong belief in the prior art that low molecular weight polyphenylene ether resins should be avoided where high impact resistance is desired.